In general, a color image forming apparatus uses a developing material (toner) to form a visible image (a toner image) from an electrostatic latent image that has been formed on a photosensitive drum, and transfers the formed visible image to an intermediate transfer member by a first transfer. The primary transfer is executed in the order of the colors used, e.g., yellow, magenta, cyan and black. This means that a plurality of toner images of different colors are transferred to the intermediate transfer member in a multiple-transfer operation. The color image forming apparatus then transfers the multiple-transferred toner images to a transfer member collectively by a secondary transfer. It should be noted that a color image forming apparatus having just one photosensitive drum and a color image forming apparatus having a plurality of photosensitive drums exist.
In an image forming apparatus of this kind, a so-called “density shift” occurs for every color owing to amount of toner remaining in a developing device and a fluctuation in transfer characteristic ascribable to atmospheric temperature. Further a so-called “color misregistration” also can occur in an image forming apparatus that uses a plurality of photosensitive drums. Color misregistration is caused by the fact that the positions at which toner images of the respective colors are formed do not coincide owing to mechanical installation error between the photosensitive drums, an error in optical path lengths of the laser light beams and a change in these optical paths.
In general, a “density shift” is adjusted automatically by the following procedure: First, a reference pattern based upon a toner image is formed on a photosensitive drum or intermediate transfer belt. Next, the density of the toner image that has been formed is sensed by a photosensor. Process conditions and a correction value of a gamma characteristic are then controlled automatically in such a manner that the result of sensing density becomes a prescribed value. As a result, image density can be stabilized.
“Color misregistration”, on the other hand, is adjusted automatically by the following procedure: First, a reference pattern that has been formed on the intermediate transfer member is read by a photosensor placed in close proximity to a photosensitive drum situated immediately downstream. Based upon the result of reading the reference pattern, color misregistration on the intermediate transfer member is then sensed for every color formed by each image forming portion. The output timing of the image signal to be printed and the image signal itself are electrically adjusted automatically so as to cancel out this color misregistration. It should be noted that the optical path length and the optical path also may be adjusted automatically by driving a bending mirror provided in the optical path of the laser beam.
With such automatic adjustment (see the specification of Japanese Patent No. 3450402), it is usually necessary to perform a maximum density correction and gray scale correction on a color-by-color basis. Further, in relation also to automatic adjustment of color misregistration, it is necessary to form a reference pattern a plurality of times in order to reduce error such as drive-system eccentricity. As a consequence, executing the automatic adjustment can take several minutes.
Furthermore, when images are formed successively on a plurality of sheets of paper, the automatic adjustments are executed from the end of image formation on a certain sheet of paper to the beginning of image formation on the next sheet of paper. In this case, formation of the next image cannot be performed until the automatic adjustments are completed. This results in a decline in productivity and requires that the operator wait.
The specification of Japanese Patent Application Laid-Open No. 2002-91096 proposes an automatic adjustment method that is capable of changing the number of image-pattern formations in accordance with the intervals at which a plurality of sheets of paper are transported.
However, an increase in the speed of image forming systems and an improvement in the productivity thereof are accompanied by a shortening in the transport interval of paper sheets. With the method described in Japanese Patent Application Laid-Open No. 2002-91096, however, there is the danger that the reference pattern cannot be formed satisfactorily.
An object of the present invention is to solve this problem and at least one other problem of the prior art. Other problems of the prior art will be understood through a reading of the entire specification.